The invention relates to a pressure control valve for a fuel tank, comprising a valve housing fitted on the fuel tank or on its filler neck, and a valve body which is spring-loaded in the direction of a rotationally symmetrical sealing surface. Valves of this type are required as a safety valve in order, firstly, to prevent the emission of fuel vapors, and, secondly, when there is a rise in pressure (for example because of a rise in temperature when exposed to the sun), to prevent too great a rise in pressure in the interior of the fuel tank. In this case, the emission limits, which are becoming ever stricter, require maximum imperviousness of the valve in the closed state; in particular also imperviousness to fuel vapors. The latter can flow more easily past a sealing ring than liquids and can also pass through by diffusion.
Pressure control valves of the generic type which are known in practice do not satisfy these requirements. Conventional methods for improving the imperviousness influence the response pressure and impede a prompt response. Therefore, the object of the invention is to provide pressure control valves which combine a maximum sealing effect with an unobstructed and reliable response with the requirements in terms of costs and space being as low as possible.
The foregoing object is attained, according to the invention, wherein the valve body comprises a foot part and a plate, the foot part being guided displaceably in the valve housing in the axis of symmetry, and the plate has, in a staggered manner, a first groove and a second groove for holding a first sealing ring and a second sealing ring, the two sealing rings bearing against the sealing surface when the valve is closed and being flowed around in series when the valve is open.
The two staggered sealing rings have the effect, first of all, that it is always only atmospheric pressure which acts on the second, outer sealing ring when the valve is closed, even when an increased internal pressure of the tank is acting on the first sealing ring. As a result, even when the two sealing rings have the same structure, the sealing effect of the outer one is greater.
The foot part, which is guided concentrically in the valve housing in the axis of symmetry, ensures a ready response, even after a very long period of not responding, without becoming caught. The addition of a second sealing ring is not only a simple duplication. It creates the possibility of providing an effective seal against media of various states of aggregation (liquid, vapor, air). The second sealing ring is protected by the first sealing ring against contact with liquid fuel, and can thus consequently be designed as a gas seal or vapor seal. For this purpose, it can be designed to be harder than the first one (claim 4), which, in the simplest case, results in a graduation of the contact pressure, or can consist of a material of high permeation resistance, which is possibly less suitable as a liquid seal.
By means of the second sealing ring, which is recessed into the groove, a narrow gap can be set which reduces the contact surface between the second sealing ring and the fuel vapor to a minimum. Since the quantity which diffuses in is proportional to the contact surface, the quantity which permeates is thereby further considerably reduced. By suitable coordination of the groove depths and dimensions of the sealing rings, the sealing effect of the two staggered sealing ring arranged xe2x80x9cin seriesxe2x80x9d can be coordinated optimally with each other.
A particularly neat guidance of the plate and a compact constructional form of the valve are achieved in that the valve housing comprises a housing case and a housing cover, on which housing cover a guide for the foot part is formed, and in that a compression spring is supported at one end on the housing cover and at the other end on the plate. In a particularly simple and inexpensive embodiment, the rotationally symmetrical sealing surface is formed on the fuel tank or on its filler neck.
In a preferred embodiment, the plate is conical, the first sealing ring, which faces the interior of the fuel tank, having, with respect to the axis of symmetry, a smaller radius than the second sealing ring (the same applies, of course, for the first and second grooves), and the rotationally symmetrical sealing surface is likewise conical.
In a particularly effective development of the invention, the first sealing ring lies in a groove formed in a further plate, and can be shifted with respect to the plate in the direction of the axis of symmetry. This can be brought about, for example, by a deformable design and dimensioning of the further plate.
In a further refinement of this development, the further plate can be displaced with respect to the plate in the direction of the axis of symmetry, and a further compression spring is supported at its one end on the plate and at its other end on the further plate. By making the two sealing rings independent, each with its own plate, not only is an even better optimization of the sealing effects of the two sealing rings obtained. Moreover, a buffer space is produced between the two plates, in which pressure peaks are reduced and a relatively large pressure effect surface of the plate [lacuna] with the second sealing ring.
A very practical design is achieved in that a groove for a sealing ring, or for both sealing rings, is bounded on the side closer to the axis of symmetry by a retaining element, a cover clipped onto the plate or a ring. As a result, the respective sealing ring is fastened on the or its plate, and thus can also be interchanged without being overstretched. Above all, however, the sealing surface of the plate can thus be injection molded in a relatively simply mold and, in addition, can be made without any burrs.
Furthermore, the sealing surface can be part of the case of the valve housing. The completely fitted and tested valve can therefore be fitted on the vehicle tank or on its filling pipe.
In an advantageous development of the valve according to the invention, the housing cover has a cylindrical wall part in which radially protruding, resilient tongues are formed, said tongues engaging in corresponding recesses in the housing case. This simplifies construction and installation and provides a reliable connection.